This invention relates to high-field double-pancake coils and a method of winding, and particularly to superconducting magnet coils with graded conductor.
There are only two basic methods of winding: layer winding and pancake winding. The most common example of layer winding can be seen on an ordinary spool of thread: thread (or conductor) is wound about a cylindrical core by placing one turn next to another, proceeding down the core surface until a layer is formed covering the core. The second layer is wound on top of the first layer and proceeds up the core surface until a second layer is formed, and so on. In pancake winding, conductor is wound about a cylindrical core placing one turn on top of each preceding turn to form a plane of turns perpendicular to the axis of the cylindrical core. In double-pancake winding, two pancakes are formed. After completion of the first pancake, the conductor is wound along the axis at the outermost turn to start the second pancake next to the first. For the second pancake, the conductor spirals inward to the core, one turn nested within another. It can readily be seen that the double-pancake can also be wound starting from the outermost turn of the first pancake first spiralling inward, winding along the axis at the innermost turn to start the second pancake, then spiralling outward. Depending on the allowability of joints, it may be convenient to wind individual double pancakes or even pancakes separately (by spiralling inward or by spiralling outward or one of each), and then connect them with electrical joints. This method can be repeated any number of times to form a coil having a series of pancakes. The double-pancake method is particularly useful for winding superconducting magnets, because conductor joints can be easily formed on the outside of the coil and because the magnet can be manufactured in modular fashion with several double pancakes being wound at the same time and then brought together at final assembly.
The design of superconducting magnets offers new problems to overcome. Superconductors which are used in magnets encounter a natural limitation: the higher the local magnetic field to which the superconductor is exposed, the lower the current density allowed in the superconductor. A current density which is above the critical current density causes the superconductor to quench or become normal. Since the innermost turn of any magnet is exposed to the highest magnetic field and the outermost turn is exposed to a smaller field, the critical current density in the innermost (high-field) turns is typically 2 to 5 times less than the critical current density in the outermost (low-field) turns. If a magnet is wound of a single size and material conductor, then the current density in the magnet is limited to the current density in the high-field turns, wasting valuable conductor in the lower field regions. This is a severe penalty in large magnets in terms of cost, material, and space occupied by the windings.
Consequently, many designs of high-field superconducting magnets use more than one grade of superconductor (the grade of a superconductor is the amount of high-field it can withstand before going normal). For example, the coil winding may be divided into three regions: high, medium, and low fields. Since the maximum field for each region is different and since grading is commonly achieve by varying the current density through the conductor, different conductors having different current densities are used in each region: high current density conductor in the low field region, medium current density conductor in the medium field region, and low current density conductor in the high field region. Since all of the turns are connected electrically in series, they all carry the same current. For these turns grading may be achieved by varying the crossn-sectional area of the conductor (current density grading). Grading is also achieved by changing the conductor material (conductor material grading). Graded superconductor is particularly convenient for layer wound magnets. It is also possible to grade the conductor in a double-pancake winding if several sizes of conductor are available and if electrical joints between the different grades of conductor are allowed within the winding. However, grading is not particularly convenient for certain pancake wound magnets.
One promising superconductor for nuclear fusion applications (e.g. toroidal field coils) is a superconducting material such as Nb.sub.3 Sn enclosed in a conduit through which a cryogenic coolant such as supercritical helium can flow. This is referred to generally as "forced flow conductor". One way of winding such a conductor in double-pancake form uses large plates with shelves provided to support the conductor. Presently, forced flow conductor double-pancake windings are not graded because of the extreme difficulty in providing electrical joints within the winding.
An important design criteria for toroidal field coils such as are used on a tokamak machine is the space in the center of the machine. All the torodial field coils on a tokamak machine meet in the center of the machine where space is a premium. Reducing the size of the toroidal field coils could save millions of dollars in the cost of a fusion reactor. It has been estimated that for a typical $1 billion superconducting tokamak, for every centimeter by which the radial thickness of the inner leg of the toroidal field coils is reduced (which reduces the major radius of the plasma by that amount), the overall machine cost is reduced by $3.5 million.
Therefore, it is an object of the present invention to provide a double-pancake coil using graded conductor with electrical joints outside the winding.
It is another object of the present invention to provide a coil which minimizes the space required for the inner legs of a toroidal field coil.
It is yet another object of the present invention to provide a method of winding a double-pancake coil which reduces the size of the coil.
It is still another object of the present invention to provide a method of winding coils which improves reliability due to the absence of electrical joints within the windings.
It is also an object of the present invention to provide an improved superconducting double-pancake coil using graded conductor with no internal electrical joints.
It is another object of the present invention to provide a double-pancake coil using graded forced flow conductor without internal electrical joints.
Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention.